This project consists of a study of the two major components related to the generation of the electrocardiogram namely the sources and the volume conductor field. In regard to the study of sources, simultaneous recording of potentials at ninety electrodes placed intramurally and epicardially is planned utilizing the time-multiplex electronics whose construction is now complete. By analysis of these signals both temporally and, more importantly, spatially one can obtain a quantitative description of the underlying sources. One objective is to relate macroscopic sources to cellular behavior (including all histology and fiber orientation) while another is to characterize the macroscopic sources quantitatively. The effect of the volume conductor field on the resultant electrocardiogram has been considered through an analytic eccentric spheres model of heart and torso which consists of five separate regions. This is permitting analysis of the effect of conductivity and geometry variation of a somewhat realistic nature to be determined. One important outcome is that the effect of changes in one parameter may depend on interaction with all the remaining parameters that characterize the heart-torso. It is planned to study the effect of variations in cardiac tissue, taking into account the complex fiber geometry, on the volume conductor properties of the heart - as an extension to this work.